Specific structural parts of the small intestinal mucosa that are barriers to intestinal drug absorption will be identified. Potential rate limiting barriers include an aqueous stagnant layer, the apical membrane, the contents of the absorbing cell, the basal and basement membranes, and subepithelial blood flow. The hypothesis is that several rate limiting barriers exist, but that only one or two barriers are operative for a particular drug; whether a barrier is operative depends on the physicochemical properties of the drug. The methods involve a kinetic analysis of the disappearance of several test solutes from a segment of the perfused, in situ rat jejunum. The test solutes span a broad range of intestinal permeability, molecular weight, oil/water partition coefficient, and molecular shape. The methods utilize a pharmacokinetic analysis of the disappearance of the test permeants from a solution recirculated through the intestinal segment. The test permeants will be administered as an impulse or as an infusion into the perfusate and the analysis will focus on the transient behavior of the concentration of permeant in the perfusate. In other studies, perfusate will be pumped on a single pass through an in situ segment of jejunum and the permeant will be administered at the entrance to the intestine as an impulse or as a sine wave. The shape of the impulse or sine wave exiting the segment will be subjectd to residence time analysis or to frequency domain (phasor) analysis, respectively. These analyses of the transient behavior of the test solutes after the various modes of input will allow identification and characterization of the various barriers that limit the rate of intestinal absorption of drugs.